This invention relates to the recovery of .epsilon.-caprolactam, as described, amongst others, in the Netherlands Patent Applications 7,106,341 and 7,106,343. According to these processes .epsilon.-caprolactam is extracted using a water-immiscible organic solvent for the lactam from a lactam-containing starting solution which is still highly acid. The starting solution is obtained by neutralizing about half of the sulfuric acid content in a rearrangement mixture containing lactam and sulfuric acid with the aid of ammonia water or an ammonium sulfate solution which forms an aqueous solution which, in addition to the lactam, mainly contains ammonium hydrogen sulfate and possibly a small amount of free sulfuric acid or ammonium sulfate, depending on the degree of neutralization of the rearrangement mixture. The organic solvent is freed from lactam by evaporation and back-extraction with water and the organic solvent is again used for the extraction of lactam from the highly acid starting solution.
Processing of the thus-obtained lactam in water solution is additionally conducted in the usual way by evaporation of water, first at atmospheric pressure until a concentration of approximately 90% by weight of lactam has been reached, and subsequently under vacuum conditions to evaporate the remaining water.
For further purification of the product it is customary to have the lactam distil over vacuo. Although the lactam reocvered in this way already of sufficient purity to satisfy various strict specification demands made on lactam intended for polymerization into nylon, such as requirements in relation to the melting point, color, clearness and permanganate number, even so the thus-treated lactam generally does not satisfy the customary alkalinity requirement.
This requirement specifies that the alkalinity, expressed in meq per kg of lactam, shall be below 0.05. The alkalinity is determined by dissolving 40 grams of lactam in distilled water, then increasing the volume to 200 ml and titrating the solution with 0.01 N of hydrochloric acid in the presence of a mixing-indicator which changes in the pH range of 5-6; preferably the "Tashiro" indicator is used.
In the lactam recovered in the manner described above, starting from different samples of lactam-sulfuric acid rearrangement mixture, invariably an alkalinity of more than 0.05 and generally an alkalinity as high as about 0.1 was found.